The first trial testing the CIA's controversial practice of extraordinary rendition opened in a Milan court today, hours ahead of a visit to Italy by the US president, George Bush.

The trial got under way with empty cages lining two walls of the courtroom. Twenty-six US citizens - all but one believed to be intelligence operatives - went on trial in absentia, accused of abducting a terrorist suspect from a Milan street in spring 2003 and flying him to Egypt where he says he was tortured under interrogation.

Though the start of the trial coincides with Mr Bush's arrival for talks with the Pope and Italy's prime minister and president, it is thought judicial proceedings will not begin in earnest for some time. The Italian government has asked Italy's highest court to throw out the indictments against the Americans accused of kidnapping Osama Moustafa Hassan Nasr four years ago. The constitutional court is expected to consider that and another similar appeal in the autumn, and participants in the trial said they expected the defence to request postponement of the trial until after the Italian high court's ruling.

In addition to the Americans, seven Italians were also indicted in the case, including Nicolo Pollari, the former chief of military intelligence. Mr Pollari, who was not at court, has denied any involvement by Italian intelligence in the abduction.

Mr Pollari's lawyer, Titta Madia, said today he would ask for an immediate suspension of the proceedings pending the constitutional court ruling, so that both sides would know which evidence could be used.

"It is in our interest that the trial proceeds as quickly as possible," Mr Madia told reporters before the hearing began. "In general, Pollari would like to arrive at an acquittal as soon as possible, but we need to have certainty about which evidence we can use."

Italian prosecutors say Mr Nasr, who is suspected of recruiting fighters for radical Islamic causes but who had not been charged with any crime at the time of his disappearance, was snatched in an Italian street, bundled into a van and driven to a US military base in northern Italy. From there, prosecutors say the CIA flew him via Germany to Egypt, where he says he was tortured with electric shocks, beatings, rape threats and genital abuse.

Mr Nasr's lawyer travelled from Cairo to attend the opening. Prosecutors have included the cleric in a list of more than 120 witnesses.

Lawyers for Mr Pollari have named on their witness list Italy's prime minister at the time, Silvio Berlusconi, and the current prime minister, Romano Prodi, lawyers involved in the case said. The same request was denied by a different judge during the preliminary hearing phase. Mr Berlusconi says the trial is a mistake that could expose secrets of international espionage.

The 26 Americans have left Italy, and a senior US official has said they would not be turned over for prosecution even if the Italian government requested it.

The trial comes at an awkward time for Mr Prodi, an increasingly unpopular leader just a year after taking office. He is trying to keep fractious coalition partners united behind him and away from the street protests against Mr Bush due to be staged tomorrow.

Pacifists in his coalition want the prime minister to pull Italian troops out of Afghanistan and scrap a permit to expand a US military base in the northern city of Vicenza.